Topological physics has been mainly studied in linear systems. Recently, the
frontier of the study of topological phases has reached nonlinear systems.
Nonlinear effects are naturally introduced in artificial topological
systems. In this context, topological physics in nonlinear systems is
studied in mechanical[1]}, [2]}, [3]}, photonic[4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, electric circuit[12]}, [13]} and resonator[14]} systems. It is an interesting
problem how the topological phases are modified in the presence of the
nonlinear term. Furthermore, it is fascinating if there is a transition
induced by the nonlinear term.
